Multilayer electrolyte membrane

ABSTRACT

The present invention relates to a proton-conducting multilayer electrolyte membrane with a barrier layer, a process for producing it and a fuel cell containing such a membrane.

RELATED APPLICATIONS

This application is the U.S. National Stage of International ApplicationNo. PCT/EP03/04117, filed Apr. 22, 2003, published in German, and claimspriority under 35 U.S.C. § 119 or 365 to Germany Application Nos. 102 18368.6 and 102 18 367.8, both filed on Apr. 25, 2002.

DESCRIPTION

The present invention relates to a proton-conducting multilayerelectrolyte membrane, a process for producing it and a fuel cellcontaining such a membrane.

A fuel cell usually comprises an electrolyte and two electrodesseparated by the electrolyte. In the case of a fuel cell, a fuel such ashydrogen gas is supplied to one of the two electrodes and an oxidantsuch as oxygen gas is supplied to the other electrode and chemicalenergy from the oxidation of the fuel is in this way converted intoelectric energy.

The electrolyte is permeable to hydrogen ions, i.e. protons, but not toreactive gases such as the hydrogen gas and the oxygen gas.

A fuel cell generally has a plurality of single cells known as MEUs(membrane-electrode units) which each comprise an electrolyte and twoelectrodes separated by the electrolyte.

Electrolytes employed for the fuel cell are solids such as polymerelectrolyte membranes or liquids such as phosphoric acid. In recenttimes, polymer electrolyte membranes have attracted attention aselectrolytes for fuel cells. An in-principle distinction can be madebetween 2 categories of polymer membranes. The first category comprisescation-exchange membranes composed of a polymer framework containingcovalently bound acid groups, preferably sulphonic acid groups. Thesulphonic acid group is converted into an anion by release of a hydrogenion and therefore conducts protons. The mobility of the proton and thusthe proton conductivity is directly related to the water content. If themembrane dries out, e.g. as a result of high temperature, theconductivity of the membrane and consequently the power of the fuel celldecreases drastically. The operating temperature of fuel cellscontaining such cation-exchange membranes is thus limited to the boilingpoint of water. For this reason, perfluorosulphonic acid polymers, forexample, are used as materials for polymer electrolyte membranes. Theperfluorosulphonic acid polymer (e.g. Nafion) generally has aperfluorohydrocarbon framework, e.g. a copolymer of tetrafluoroethyleneand trifluorovinyl, and a side chain which is bound thereto and bears asulphonic acid group, e.g. a side chain having a sulphonic acid groupbound to a perfluoroalkylene group. Moistening of the fuels is animportant industrial requirement for the use of polymer electrolytemembrane fuel cells (PEMFCS) in which conventional, sulphonatedmembranes such as Nafion are used.

A second category which has been developed comprises polymer electrolytemembranes composed of complexes of basic polymers and strong acids.Thus, WO 96/13872 and the corresponding U.S. Pat. No. 5,525,436 describea process for preparing a proton-conductive polymer electrolytemembrane, in which a basic polymer such as polybenzimidazole is treatedwith a strong acid such as phosphoric acid, sulphuric acid, etc.

A fuel cell in which such a polymer electrolyte membrane is used has theadvantage that it can be operated without moistening and at temperaturesof 100° C. or above.

In J. Electrochem. Soc., volume 142, No. 7, 1995, pp. L121-L123, dopingof a polybenzimidazole in phosphoric acid is described.

In the case of the basic polymer membranes known from the prior art, themineral acid used for achieving the required proton conductivity(usually concentrated phosphoric acid) is either used after shaping or,as an alternative, the basic polymer membrane is produced directly frompolyphosphoric acid as in the German patent applications Nos.10117686.4, 10144815.5 and 10117687.2. Here, the polymer serves assupport for the electrolyte consisting of the highly concentratedphosphoric acid or polyphosphoric acid. The polymer membrane herefulfils further essential functions, in particular it has to have a highmechanical stability and serve as separator for the two fuels mentionedat the outset.

An important advantage of such a membrane doped with phosphoric acid isthe fact that this system can be operated at temperatures above 100° C.without moistening of the fuels which would otherwise be necessary. Thisis due to the ability of phosphoric acid to be able to transport protonswithout additional water by means of the Grotthus mechanism (K.-D.Kreuer, Chem. Mater. 1996, 8, 610-641). The ability to operate the fuelcell system at temperatures above 100° C. results in further advantagesfor the system. Firstly, the sensitivity of the Pt catalyst to gasimpurities, in particular CO, is greatly reduced. CO is formed asby-product in the reforming of the hydrogen-rich gas fromcarbon-containing compounds, e.g. natural gas, methanol or petroleumspirit, or as intermediate in the direct oxidation of methanol. The COcontent of the fuel typically has to be less than 100 ppm attemperatures of <100C. However, at temperatures in the range 150-200°C., 10,000 ppm or more of CO can also be tolerated (N. J. Bjerrum et.al. Journal of Applied Electrochemistry, 2001, 31, 773-779). This leadsto substantial simplification of the upstream reforming process and thusto cost reductions for the overall fuel cell system.

A great advantage of fuel cells is the fact that in the electrochemicalreaction the energy of the fuel is converted directly into electricenergy and heat. Water is formed as reaction product at the cathode.Heat is thus produced as by-product of the electrochemical reaction. Inthe case of applications in which only the electric power is utilizedfor driving electric motors, e.g. for automobile applications, the heathas to be removed to avoid overheating of the system. Additional,energy-consuming equipment is therefore necessary for cooling, and thisfurther reduces the overall electrical efficiency of the fuel cell. Inthe case of stationary applications such as for central or decentralizedgeneration of electric power and heat, the heat can be utilizedefficiently by means of existing technologies, e.g. heat exchangers. Toincrease the efficiency, high temperatures are desirable. If theoperating temperature is above 100° C. and the temperature differencebetween ambient temperature and the operating temperature is large, itbecomes possible to cool the fuel cell system more efficiently or to usesmall cooling areas and dispense with additional equipment compared tofuel cells which, owing to moistening of the membrane, have to beoperated at below 100° C.

Besides these advantages, such a system has two critical disadvantages.Thus, phosphoric acid is present as an electrolyte which is not boundpermanently by ionic interactions to the basic polymer and can be washedout by water. Water is, as described above, formed at the cathode in theelectrochemical reaction. If the operating temperature is above 100° C.,the water is mostly removed as vapour through the gas diffusionelectrode and the acid loss is very small. However, if the operatingtemperature drops below 100° C., e.g. on starting up and shutting downthe cell or in part-load operation when a high current yield is sought,the water formed condenses and can lead to increased leaching of theelectrolyte, viz. highly concentrated phosphoric acid.

In the above-described mode of operation of the fuel cell, this can leadto a continual decrease in the conductivity and cell power, which canreduce the life of the fuel cell.

A further disadvantage of fuel cells in which phosphoric acid functionsas electrolyte is inhibition of the reduction reaction at the cathode,resulting in a high overvoltage. This leads to a low equilibrium restpotential and a relatively low power.

Furthermore, the known membranes doped with phosphoric acid cannot beused in the direct methanol fuel cell (DMFC). However, such cells are ofparticular interest since a methanol/water mixture is used as fuel. If aknown membrane based on phosphoric acid is used, the fuel cell failsafter quite a short time.

It is therefore an object of the present invention to provide a polymerelectrolyte membrane in which the leaching of the mineral acid isreduced or prevented and which additionally has a reduced overvoltage,in particular at the cathode. In particular, the operating temperatureshould be able to be in the extended range from <0° C. to 200° C.

A further object of the present invention was to provide a membranewhich even in operation has a low permeability to a wide variety offuels, for example hydrogen or methanol, and also displays a low oxygenpermeability.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic structure of the measurement apparatus employed tomeasure the barrier action of the cation-exchange membranes doped withphosphoric acid.

FIG. 2 is a plot of the values of pH as a function of time for differentcation-exchange

FIG. 3 is a plot of the values of pH as a function of time for differentcation-exchange membranes, corrected for the blank.

FIG. 4 is a bar plot of the amounts of acid which passed through thebarrier layer and which was retained by the barrier layers of differentcation-exchange membranes.

FIG. 5 is a plot showing the values of pH of a volume of water as afunction of time, demonstrating effectiveness of a barrier layer in anexperimental setup shown in FIG. 1.

DETAILED DESCRIPTION

The object of the invention is achieved by a multilayer membrane systemcomprising a polymer electrolyte membrane which is doped with mineralacid and is coated on at least one side with a barrier layer for themineral acid. In this configuration, the membrane doped with mineralacid performs the essential functions as separator for the fuels and theprovision of mechanical stability. The barrier layer is intended toprevent the loss of mineral acid and to reduce the overvoltage at thecathode.

A polymer electrolyte membrane according to the invention has a very lowmethanol permeability and is particularly suitable for use in a DMFC.Long-term operation of a fuel cell using many fuels such as hydrogen,natural gas, petroleum spirit, methanol or biomass is thus possible.Here, the membranes allow a particularly high activity of these fuels.As a result of the high temperatures, the methanol oxidation can occurwith high activity.

The present invention accordingly provides a multilayer electrolytemembrane comprising

-   A. a sheet-like material doped with one or more mineral acids, and-   B. at least one barrier layer which covers at least one of the two    surfaces of the material specified under A.

In the case of the sheet-like materials A, use is made of basicpolymers, mixtures of basic polymers with other polymers or chemicallyinert supports, preferably ceramic materials, in particular siliconcarbides (SiC) as are described in U.S. Pat. Nos. 4,017,664 and4,695,518. These materials are capable of transporting protons by theGrotthus mechanism.

A thermally stable and chemically inert support which is filled withphosphoric acid to achieve proton conductivity can be used as sheet-likematerial. Possible support materials are, for example, ceramic materialssuch as silicon carbide SiC (U.S. Pat. Nos. 4,017,664 and 4,695,518) orinorganic glasses. This support can, for example, be in the form of awoven fabric or a nonwoven. Furthermore, the support can also be made upof porous materials.

As chemically inert support, it is also possible to use porous organicpolymers having an open pore structure. The open pore volume is in thiscase more than 30%, preferably more than 50% and very particularlypreferably more than 70%. The glass transition temperature of theorganic base polymer of such a membrane is higher than the operatingtemperature of the fuel cell and is preferably at least 150° C., morepreferably at least 160° C. and very particularly preferably at least180° C. Such membranes are used as separation membranes forultrafiltration, gas separation, pervaporation, nanofiltration,microfiltration or haemodialysis.

Methods of producing such membranes are described in H. P. Hentze, M.Antonietti “Porous polymers and resins” in F. Schüth “Handbook of PorousSolids” pp. 1964-2013.

It is also possible to produce organic foams as chemically inertsupports. These foams can be produced by gases such as CO₂ beingliberated in the synthesis of the organic polymer or volatile liquidsbeing used. Methods of producing organic foams are described in D.Klempner, K. C. Frisch “Handbook of Polymeric Foams and Foam Technology”and F. A. Shutov Advances in Polymer Science Volume 73/74, 1985, pages63-123. Supercritical CO₂ can also be used as pore former.

A particularly advantageous support is a phase separation membranecomposed of polybenzimidazole, which can be produced as described inU.S. Pat. Nos. 4,693,824 or 4,666,996 or 5,091,087. The chemicalstability of these membranes towards phosphoric acid or polyphosphoricacid can be further improved by crosslinking by means of the methoddescribed in U.S. Pat. No. 4,634,530.

Furthermore, it is possible to use expanded polymer films such asexpanded Teflon as support materials. Methods of producingproton-conducting membranes by filling such an expanded perfluorinatedmembrane are described in U.S. Pat. No. 5,547,551.

Likewise, high-porosity thermosets which have been prepared bychemically induced phase separation can likewise be used as supportmaterials. In this process, a highly volatile solvent is added to amixture of a plurality of monomers capable of crosslinking. This solventbecomes insoluble during crosslinking and a heterogeneous polymer isformed. Evaporation of the solvent produces a chemically inert, porousthermoset which can subsequently be impregnated with phosphoric acid orpolyphosphoric acid.

A particularly useful support can be produced from inorganic materials,for example glass or materials which comprise at least one compound of ametal, a semimetal or a mixed metal or phosphorus with at least oneelement of main groups 3 to 7. The material particularly preferablycomprises at least one oxide of the elements Zr, Ti, Al or Si. Thesupport can consist of an electrically insulating material, e.g.minerals, glasses, plastics, ceramics or natural materials. The supportpreferably comprises specific woven fabrics, nonwovens or porousmaterials composed of high-temperature-resistant and highlyacid-resistant fused silica or glass. The glass preferably comprises atleast one compound from the group consisting of SiO₂, Al₂O₃ or MgO. In afurther variant, the support comprises Woven fabrics, nonwovens orporous materials composed of Al₂O₃, ZrO₂, TiO₂, Si₃N₄ or SiC ceramic. Tokeep the total resistance of the electrolyte membrane low, this supportpreferably has a very high porosity but also a low thickness of lessthan 1000 μm, preferably less than 500 μm and very particularlypreferably less than 200 μm. Preference is given to using supports whichcomprise woven fibres of glass or fused silica, with the woven fabricspreferably being composed of 11-tex yarns with 5-50 warp threads or weftthreads and preferably 20-28 warp threads and 28-36 weft threads.Particular preference is given to 5.5-tex yarns with 10-50 warp threadsor weft threads and preferably 20-28 warp threads and 28-36 weftthreads.

As indicated above, supports comprising woven fabrics, nonwovens orporous materials can be used. Porous materials based on, in particular,organic or inorganic foams are known.

Preferred supports are permeable to mineral acids without a barrierlayer. This property can be confirmed by the experiment on barrieraction presented in the examples. According to a particular aspect ofthe present invention, at least 5% of a mineral acid present in thesheet-like structure is liberated within 1 hour if the sheet-likematerial is exposed to a large excess of water (an at least 100-foldamount, based on the weight of the sheet) having a temperature of 180°C.

Depending on the field of application, the sheet-like structure A) canbe stable to high temperatures. Stable to high temperatures means thatthe support is stable at a temperature of at least 150° C., preferablyat least 200° C. and particularly preferably at least 250° C. Stablemeans that the significant properties of the support are retained. Thus,no change in the mechanical properties or in the chemical compositionoccurs on exposure of the sheet-like material for at least 1 hour.

In general, the support is chemically inert. Chemically inert means thata sheet-like material doped with a mineral acid is chemically stable.Chemically stable means that the material is not decomposed by the acid.Thus, the material after 100 hours displays at least 95% of themechanical properties displayed by the material at the beginning of themeasurement. This applies, for example, to the modulus of elasticity andthe microhardness.

As basic polymer membrane doped with mineral acid, it is possible to usevirtually all known polymer membranes in which the protons aretransported without additional water, e.g. by means of the Grotthusmechanism.

A basic polymer for the purposes of the present invention is a basicpolymer having at least one nitrogen atom in a repeating unit.

The repeating unit in the basic polymer preferably contains an aromaticring having at least one nitrogen atom. The aromatic ring is preferablya five- or six-membered ring which has from one to three nitrogen atomsand can be fused with another ring, in particular another aromatic ring.

Polymers based on polyazole generally comprise recurring azole units ofthe general formula (I) and/or (II) and/or (III) and/or (IV) and/or (V)and/or (VI) and/or (VII) and/or (VIII) and/or (IX) and/or (X) and/or(XI) and/or (XII) and/or (XIII) and/or (XIV) and/or (XV) and/or (XVI)and/or (XVI) and/or (XVII) and/or (XVIII) and/or (XIX) and/or (XX)and/or (XXI) and/or (XXII)

where

-   the radicals Ar are identical or different and are each a    tetravalent aromatic or heteroaromatic group which may be monocyclic    or polycyclic,-   the radicals Ar¹ are identical or different and are each a divalent    aromatic or heteroaromatic group which may be monocyclic or    polycyclic,-   the radicals Ar² are identical or different and are each a divalent    or trivalent aromatic or heteroaromatic group which may be    monocyclic or polycyclic,-   the radicals Ar³ are identical or different and are each a trivalent    aromatic or heteroaromatic group which may be monocyclic or    polycyclic,-   the radicals Ar⁴ are identical or different and are each a trivalent    aromatic or heteroaromatic group which may be monocyclic or    polycyclic,-   the radicals Ar⁵ are identical or different and are each a    tetravalent aromatic or heteroaromatic group which may be monocyclic    or polycyclic,-   the radicals Ar⁶ are identical or different and are each a divalent    aromatic or heteroaromatic group which may be monocyclic or    polycyclic,-   the radicals Ar⁷ are identical or different and are each a divalent    aromatic or heteroaromatic group which may be monocyclic or    polycyclic,-   the radicals Ar⁸ are identical or different and are each a trivalent    aromatic or heteroaromatic group which may be monocyclic or    polycyclic,-   the radicals Ar⁹ are identical or different and are each a divalent    or trivalent or tetravalent aromatic or heteroaromatic group which    may be monocyclic or polycyclic,-   the radicals Ar¹⁰ are identical or different and are each a divalent    or trivalent aromatic or heteroaromatic group which may be    monocyclic or polycyclic,-   the radicals Ar¹¹ are identical or different and are each a divalent    aromatic or heteroaromatic group which may be monocyclic or    polycyclic,-   the radicals X are identical or different and are each oxygen,    sulphur or an amino group which bears a hydrogen atom, a group    containing 1-20 carbon atoms, preferably a branched or unbranched    alkyl or alkoxy group, or an aryl group as further radical,-   the radicals R are identical or different and are each hydrogen, an    alkyl group or an aromatic group and-   n, m are each an integer greater than or equal to 10, preferably    greater than or equal to 100.

Aromatic or heteroaromatic groups which are preferred according to theinvention are derived from benzene, naphthalene, biphenyl, diphenylether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulphone, thiophene, furan, pyrrole, thiazole, oxazole, imidazole,isothiazole, isoxazole, pyrazole, 1,3,4-oxadiazole,2,5-diphenyl-1,3,4-oxadiazole, 1,3,4-thiadiazole, 1,3,4-triazole,2,5-diphenyl-1,3,4-triazole, 1,2,5-triphenyl-1,3,4-triazole,1,2,4-oxadiazole, 1,2,4-thiadiazole, 1,2,4-triazole, 1,2,3-triazole,1,2,3,4-tetrazole, benzo[b]thiophene, benzo[b]furan, indole,benzo[c]thiophene, benzo[c]furan, isoindole, benzoxazole, benzothiazole,benzimidazole, benzisoxazole, benzisothiazole, benzopyrazole,benzothiadiazole, benzotriazole, dibenzofuran, dibenzothiophene,carbazole, pyridine, bipyridine, pyrazine, pyrazole, pyrimidine,pyridazine, 1,3,5-triazine, 1,2,4-triazine, 1,2,4,5-triazine, tetrazine,quinoline, isoquinoline, quinoxaline, quinazoline, cinnoline,1,8-naphthyridine, 1,5-naphthyridine, 1,6-naphthyridine,1,7-naphthyridine, phthalazine, pyridopyrimidine, purine, pteridine orquinolizine, 4H-quinolizine, diphenyl ether, anthracene, benzopyrrole,benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine,benzopyrazidine, benzopyrimidine, benzotriazine, indolizine,pyridopyridine, imidazopyrimidine, pyrazinopyrimidine, carbazole,aciridine, phenazine, benzoquinoline, phenoxazine, phenothiazine,acridizine, benzopteridine, phenanthroline and phenanthrene, which mayalso be substituted.

Here, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can have any substitutionpattern; in the case of phenylene, for example, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸,Ar⁹, Ar¹⁰, Ar¹¹ can be ortho-, meta- or para-phenylene. Particularlypreferred groups are derived from benzene and biphenyls, which may alsobe substituted.

Preferred alkyl groups are short-chain alkyl groups having from 1 to 4carbon atoms, e.g. methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. The alkylgroups and the aromatic groups may be substituted.

Preferred substituents are halogen atoms such as fluorine, amino groups,hydroxy groups or short-chain alkyl groups such as methyl or ethylgroups.

Preference is given to polyazoles which comprise recurring units of theformula (I) and in which the radicals X are identical within a recurringunit.

The polyazoles can in principle also be made up of different recurringunits which differ, for example, in their radical X. However, thepolyazole preferably has only identical radicals X in a recurring unit.

Further preferred polyazole polymers are polyimidazoles,polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles, poly(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

In a further embodiment of the present invention, the polymer comprisingrecurring azole units is a copolymer or a blend comprising at least twounits of the formulae (I) to (XXII) which differ from one another. Thepolymers can be in the form of block copolymers (diblock, triblock),random copolymers, periodic copolymers and/or alternating polymers.

In a particularly preferred embodiment of the present invention, thepolymer comprising recurring azole units is a polyazole made up only ofunits of the formula (I) and/or (II).

The number of recurring azole units in the polymer is preferably greaterthan or equal to 10. Particularly preferred polymers contain at least100 recurring azole units.

For the purposes of the present invention, polymers comprising recurringbenzimidazole units are preferred. Some examples of extremelyadvantageous polymers comprising recurring benzimidazole units have thefollowing formulae:

where n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100.

Further preferred polyazole polymers are polyimidazoles,polybenzimidazole ether ketone, polybenzothiazoles, polybenzoxazoles,polytriazoles, polyoxadiazoles, polythiadiazoles, polypyrazoles,polyquinoxalines, poly(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

Preferred polyazoles have a high molecular weight. This applies inparticular to the polybenzimidazoles. Measured as intrinsic viscosity,it is in the range from 0.3 to 10 dl/g, preferably from 1 to 5 dl/g.

Particular preference is given to Celazole from Celanese. The propertiesof the polymer film and polymer membrane can be improved by sieving thestarting polymer, as described in the German patent application No.10129458.1.

The polymer film based on basic polymers which is used for doping cancontain further additions of fillers and/or auxiliaries. In addition,the polymer film can be modified in further ways, for example bycrosslinking as in the German patent application No. 10110752.8 or in WO00/44816. In a preferred embodiment, the polymer film comprising a basicpolymer and at least one blend component which is used for dopingadditionally contains a crosslinker as described in the German patentapplication No. 10140147.7. An important advantage of such a system isthe fact that higher degrees of doping and thus higher conductivitiescombined with satisfactory mechanical stability of the membrane can beachieved.

Apart from the abovementioned basic polymers, it is also possible to usea blend of one or more basic polymers with a further polymer. The blendcomponent essentially has the task of improving the mechanicalproperties and reducing the material costs. A preferred blend componentis polyether sulphone as described in the German patent application No.10052242.4.

Preferred polymers which can be used as blend component include, interalia, polyolefins such as poly(chloroprene), polyacetylene,polyphenylene, poly(ρ-xylylene), polyarylmethylene, polyarmethylene,polystyrene, polymethylstyrene, polyvinyl alcohol, polyvinyl acetate,polyvinyl ether, polyvinylamine, poly(N-vinylacetamide),polyvinylimidazole, polyvinylcarbazole, polyvinylpyrrolidone,polyvinylpyridine, polyvinyl chloride, polyvinylidene chloride,polytetrafluoroethylene, polyhexafluoropropylene, copolymers of PTFEwith hexafluoropropylene, with perfluoropropyl vinyl ether, withtrifluoronitrosomethane, with sulphonyl fluoride vinyl ether, withcarbalkoxyperfluoroalkoxy vinyl ether, polychlorotrifluoroethylene,polyvinyl fluoride, polyvinylidene fluoride, polyacrolein,polyacrylamide, polyacrylonitrile, polycyanoacrylates,polymethacrylimide, cycloolefinic copolymers, in particular those ofnorbornene;

polymers having C—O bonds in the main chain, for example polyacetal,polyoxymethylene, polyether, polypropylene oxide, polyepichlorohydrin,polytetrahydrofuran, polyphenylene oxide, polyether ketone, polyesters,in particular polyhydroxyacetic acid, polyethylene terephthalate,polybutylene terephthalate, polyhydroxybenzoate, polyhydroxypropionicacid, polypivalolactone, polycaprolactone, polymalonic acid,polycarbonate;polymers having C—S bonds in the main chain, for example polysulphideethers, polyphenylene sulphide, polyether sulphone; polymers having C—Nbonds in the main chain, for example polyimines, polyisocyanides,polyetherimine, polyaniline, polyamides, polyhydrazides, polyurethanes,polyimides, polyazoles, polyazines;liquid-crystalline polymers, in particular Vectra and also inorganicpolymers, for example polysilanes, polycarbosilanes, polysiloxanes,polysilicic acid, polysilicates, silicones, polyphosphazenes andpolythiazyl.

For use in fuel cells having a long-term use temperature above 100° C.,preference is given to blend polymers which have a glass transitiontemperature or Vicat softening temperature VST/A/50 of at least 100° C.,preferably at least 150° C. and very particularly preferably at least180° C.

Preference is here given to polysulphones having a Vicat softeningtemperature VST/A/50 of from 180° C. to 230° C.

Preferred polymers include polysulphones, in particular polysulphonehaving an aromatic in the main chain. According to a particular aspectof the present invention, preferred polysulphones and polyethersulphones have a melt volume rate MVR 300/21.6 of less than or equal to40 cm³/10 min, in particular less than or equal to 30 cm³/10 min andparticularly preferably less than or equal to 20 cm³/10 min, measured inaccordance with ISO 1133.

To improve the use properties further, the sheet-like material cancontain fillers, in particular proton-conducting fillers.

Nonlimiting examples of proton-conducting fillers are

-   sulphates such as: CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄, NaHSO₄,    KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,-   phosphates such as Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃, UO₂PO₄.3H₂O,    H₈UO₂PO₄, Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄, LiH₂PO₄, NH₄H₂PO₄,    CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄, H₅Sb₅P₂O₂₀,-   polyacids such as H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW₁₂O₄₀.nH₂O    (n=21-29), H_(x)WO₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃,    HTiNbO₅, HTiTaO₅, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄-   selenites and arsenides such as (NH₄)₃H(SeO₄)₂, UO₂AsO₄,    (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂, Rb₃H(SeO₄)₂,-   oxides such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃-   silicates such as zeolites, zeolites(NH₄ ⁺), sheet silicates,    framework silicates, H-natrolites, H-mordenites, NH₄-analcines,    NH₄-sodalites, NH₄-gallates, H-montmorillonites-   acids such as HClO₄, SbF₅-   fillers such as carbides, in particular SiC, Si₃N₄, fibres, in    particular glass fibres, glass powders and/or polymer fibres,    preferably ones based on polyazoles.

These additives can be present in customary amounts in theproton-conducting polymer membrane, but the positive properties such ashigh conductivity, long life and high mechanical stability of themembrane should not be impaired too much by addition of excessivelylarge amounts of additives. In general, the membrane comprises not morethan 80% by weight, preferably not more than 50% by weight andparticularly preferably not more than 20% by weight, of additives.

To produce the polymer film, the polymer constituents are firstlydissolved or suspended as described in the above-cited patentapplications, for example DE No. 10110752.8 or WO 00/44816, andsubsequently used for producing the polymer films. Furthermore, thepolymer films as described in DE No. 10052237.8 can be producedcontinuously.

As an alternative, film formation can be carried out by the processdescribed in the Japanese patent application No. Hei 10-125560.

Here, the solution is poured into a cylinder having a cylindricalinterior surface and the cylinder is subsequently set into rotation. Atthe same time, the solvent is allowed to evaporate by means of thecentrifugal force caused by the rotation, so that a cylindrical polymerfilm of largely uniform thickness is formed on the interior surface ofthe cylinder.

The basic polymer having a uniform matrix can be formed by this process.

This process described in the Japanese patent application Hei 10-125560is likewise incorporated by reference into the present description.

The solvent is subsequently removed. This can be achieved by methodsknown to those skilled in the art, for example by drying.

The film of basic polymer or polymer blend is subsequently impregnatedor doped with a strong acid, preferably a mineral acid, with the film asdescribed in the German patent application No. 10109829.4 being able tobe treated beforehand. This variant is advantageous in order to rule outinteractions of the residual solvent with the barrier layer.

For this purpose, the film of basic polymer or polymer blend is dippedinto a strong acid so that the film is impregnated with the strong acidand becomes a proton-conducting membrane. For this purpose, the basicpolymer is usually dipped into a highly concentrated strong acid havinga temperature of at least 35° C. for a period of from a number ofminutes to a number of hours.

As strong acid, use is made of mineral acid, in particular phosphoricacid and/or sulphuric acid.

For the purposes of the present description, the term “phosphoric acid”refers to polyphosphoric acid (H_(n+2)P_(n)O_(3n+1)(n>1) usually has anassay calculated as P₂O₅ (acidimetric) of at least 83%, phosphonic acid(H₃PO₃), orthophosphoric acid (H₃PO₄), pyrophosphoric acid (H₄P₂O₇),triphosphoric acid (H₅P₃O₁₀) and metaphosphoric acid. The phosphoricacid, in particular orthophosphoric acid, preferably has a concentrationof at least 80 percent by weight, particularly preferably aconcentration of at least 85 percent by weight, more preferably aconcentration of at least 87 percent by weight and very particularlypreferably a concentration of at least 89 percent by weight. The reasonfor this is that as the concentration of the strong acid increases, thebasic polymer can be impregnated with a greater number of molecules ofstrong acid.

The polymer electrolyte membrane obtained, namely the complex of thebasic polymer and the strong acid, is proton-conducting. After doping,the degree of doping expressed as mole of acid per repeating unit shouldbe greater than 6, preferably greater than 8 and very particularlypreferably greater than 9.

In place of polymer membranes based on basic polymers which have beenproduced by means of classical methods, it is also possible to usepolyazole-containing polymer membranes as described in the German patentapplications Nos. 10117686.4, 10144815.5, 10117687.2. Such polymerelectrolyte membranes provided with at least one barrier layer arelikewise subject-matter of the present invention.

Accordingly, sheet-like materials according to the invention can beobtained by a process comprising the steps

-   i) preparation of a mixture comprising    -   polyphosphoric acid,    -   at least one polyazole and/or at least one compound which are/is        suitable for forming polyazoles under the action of heat as        described in step ii),-   ii) heating of the mixture obtainable as described in step i) under    inert gas to temperatures of up to 400° C.,-   iii) application of a layer to a support using the mixture as    described in step i) and/or ii),-   iv) treatment of the membrane formed in step iii).

For this purpose, one or more compounds which are suitable for formingpolyazoles under the action of heat as described in step ii) can beadded to the mixture from step i).

Mixtures comprising one or more aromatic and/or heteroaromatictetraamino compounds and one or more aromatic and/or heteroaromaticcarboxylic acids or derivatives thereof which have at least two acidgroups per carboxylic acid monomer are suitable for this purpose.Furthermore, it is possible to use one or more aromatic and/orheteroaromatic diaminocarboxylic acids for preparing polyazoles.

Suitable aromatic and heteroaromatic tetraamino compounds include, interalia, 3,3′,4,4′-tetraminobiphenyl, 2,3,5,6-tetraminopyridine,1,2,4,5-tetraminobenzole, bis(3,4,diaminodiphenyl) sulphone,bis(3,4,-diaminodiphenyl ether, 3,3′,4,4′-tetraminobenzophenone,3,3′,4,4′-tetraminodiphenylmethane and3,3′,4,4′-tetraminodiphenyldimethylmethane and also salts thereof, inparticular monohydrochloride, dihydrochloride, trihydrochloride andtetrahydrochloride derivatives thereof. Among these, particularpreference is given to 3,3′,4,4′-tetraminobiphenyl,2,3,5,6-tetraminopyridine and 1,2,4,5-tetraminobenzole.

Furthermore, the mixture A) can comprise aromatic and/or heteroaromaticcarboxylic acids. These are dicarboxylic acids and tricarboxylic acidsand tetracarboxylic acids or their esters or their anhydrides or theiracid halides, in particular their acid halides and/or acid bromides. Thearomatic dicarboxylic acids are preferably isophthalic acid,terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid,4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid,5-aminoisophthalic acid, 5-N,N-dimethylaminoisophthalic acid,5-N,N-diethylaminoisophthalic acid, 2,5-dihydroxyterephthalic acid,2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid,2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid,3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalicacid, 2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, (diphenylether)-4,4′-dicarboxylic acid, benzophenone-4,4′-dicarboxylic acid,(diphenyl sulphone)-4,4′-dicarboxylic acid, biphenyl-4,4′-dicarboxylicacid, 4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid, 4-carboxycinnamic acid, or their C1-C20-alkyl esters orC5-C1-2-aryl esters or their acid anhydrides or their acid chlorides.

The heteroaromatic carboxylic acids are heteroaromatic dicarboxylicacids and tricarboxylic acids and tetracarboxylic acids or their estersor their anhydrides. For the purposes of the present invention,heteroaromatic carboxylic acids are aromatic systems which contain atleast one nitrogen, oxygen, sulphur or phosphorus atom in the aromatic.Preference is given to pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridin-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazined icarboxylic acid, 2,4,6-pyridinetricarboxylic acid,benzimidazole-5,6-dicarboxylic acid, and also their C1-C20-alkyl estersor C5-C12-aryl esters, or their acid anhydrides or their acid chlorides.

Furthermore, the mixture i) can also contain aromatic and heteroaromaticdiaminocarboxylic acids. These include, inter alia, diaminobenzoic acid,4-phenoxycarbonylphenyl 3,′4′-diaminophenyl ether and theirmonohydrochloride and dihydrochloride derivatives.

The mixture prepared in step i) preferably comprises at least 0.5% byweight, in particular from 1 to 30% by weight and particularlypreferably from 2 to 15% by weight, of monomers for preparingpolyazoles.

According to a further aspect of the present invention, the mixtureprepared in step A) comprises compounds which are suitable for formingpolyazoles under the action of heat as described in step B), with thesecompounds being obtainable by reacting one or more aromatic and/orheteroaromatic tetraamino compounds with one or more aromatic and/orheteroaromatic carboxylic acids or derivatives thereof which contain atleast two acid groups per carboxylic acid monomer, or by reaction of oneor more aromatic and/or heteroaromatic diaminocarboxylic acids in themelt at temperatures of up to 400° C., in particular up to 350° C.,preferably up to 280° C. The compounds used for preparing theseprepolymers have been described above.

Furthermore, polyazoles can be prepared using monomers which containcovalently bound acid groups. These include, inter alia, aromatic andheteroaromatic dicarboxylic acids or derivatives thereof which have atleast one phosphonic acid group, for example2,5-dicarboxyphenylphosphonic acid, 2,3-dicarboxyphenylphosphonic acid,3,4-dicarboxyphenylphosphonic acid and 3,5-dicarboxyphenylphosphonicacid; aromatic and heteroaromatic dicarboxylic acids and derivativesthereof which contain at least one sulphonic acid group, in particular2,5-dicarboxyphenylsulphonic acid, 2,3-dicarboxyphenylsulphonic acid,3,4-dicarboxyphenylsulphonic acid and 3,5-dicarboxyphenylsulphonic acid;aromatic and heteroaromatic diaminocarboxylic acids containing at leastone phosphonic acid group, for example2,3-diamino-5-carboxyphenylphosphonic acid,2,3-diamino-6-carboxyphenylphosphonic acid and3,4-diamino-6-carboxyphenylphosphonic acid; aromatic and heteroaromaticdiaminocarboxylic acids containing at least one sulphonic acid group,for example 2,3-diamino-5-carboxyphenylsulphonic acid,2,3-diamino-6-carboxyphenylsulphonic acid and3,4-diamino-6-carboxyphenylsulphonic acid.

A polyazole membrane produced by the process described above can containthe optional components described above. These include, in particular,blend polymers and fillers. Blend polymers can, inter alia, bedissolved, dispersed or suspended in the mixture obtained as describedin step i) and/or step ii). Here, the weight ratio of polyazole topolymer (B) is preferably in the range from 0.1 to 50, more preferablyfrom 0.2 to 20, particularly preferably from 1 to 10, without thisimplying a restriction. If the polyazole is formed only in step ii), theweight ratio can be calculated from the weight of monomers for formingthe polyazole, with the compounds liberated in the condensation, forexample water, being taken into account.

To improve the use properties further, fillers, in particularproton-conducting fillers, and additional acids can additionally beadded to the membrane. The addition can, for example, be effected instep i), step ii) and/or step iii). Furthermore, these additives can, ifthey are in liquid form, also be added after the polymerizationaccording to step iv). These additives have been described above.

The polyphosphoric acid used in step i) is a commercial polyphosphoricacid as is obtainable, for example, from Riedel-de Haen. Polyphosphoricacids H_(n+2)P_(n)O_(3n+1) (n>1) usually have an assay calculated asP₂O₅ (acidimetric) of at least 83%. Instead of a solution of themonomers, a dispersion/suspension can also be produced.

In step ii) the mixture obtained in step i) is heated to a temperatureof up to 400° C., in particular 350° C., preferably up to 280° C., inparticular from 100° C. to 250° C. and particularly preferably in therange from 200° C. to 250° C. This is carried out using an inert gas,for example nitrogen or a noble gas such as neon, argon.

The mixture prepared in step i) and/or step ii) can additionally containorganic solvents. These can have a positive influence on theprocessability. Thus, for example, the rheology of the solution can beimproved so that it can be extruded or applied by doctor blade coatingmore easily.

The formation of the sheet-like structure in step iii) is carried out bymeans of measures known per se (casting, spraying, doctor blade coating,extrusion) which are known from the prior art relating to polymer filmproduction. Suitable supports are all supports which may be regarded asinert under the conditions. These supports include, in particular, filmscomposed of polyethylene terephthalate (PET), polytetrafluoroethylene(PTFE), polyhexafluoropropylene, copolymers of PTFE withhexafluoropropylene, polyimides, polyphenylene sulphides (PPS) andpolypropylene (PP). Furthermore, the membrane can also be formeddirectly on the electrode provided with a barrier layer.

The thickness of the sheet-like structure produced in step iii) ispreferably from 10 to 4000 μm, more preferably from 15 to 3500 μm, inparticular from 20 to 3000 μm, particularly preferably from 30 to 1500μm and very particularly preferably from 50 to 1200 μm.

The treatment of the membrane in step iv) is carried out, in particular,at temperatures in the range from 0° C. to 150° C., preferably attemperatures of from 10° C. to 120° C., in particular from roomtemperature (20° C.) to 90° C., in the presence of moisture or waterand/or water vapour. The treatment is preferably carried out underatmospheric pressure, but can also be carried out at superatmosphericpressure. It is important that the treatment occurs in the presence ofsufficient moisture, with the polyphosphoric acid present contributingto strengthening of the membrane by partial hydrolysis to form lowmolecular weight polyphosphoric acid and/or phosphoric acid.

The partial hydrolysis of polyphosphoric acid in step iv) leads tostrengthening of the membrane and to a decrease in the layer thicknessand formation of a membrane. The strengthened membrane generally has athickness in the range from 15 to 3000 μm, preferably from 20 to 2000μm, in particular from 20 to 1500 μm.

The upper limit to the temperature of the treatment in step iv) isgenerally 150° C. If moisture acts for an extremely short time, forexample in the case of superheated steam, this steam can also be hotterthan 150° C. The upper limit to the temperature is critically dependenton the duration of the treatment.

The partial hydrolysis (step iv) can also be carried out in temperature-and humidity-controlled chambers in which the hydrolysis can becontrolled in a targeted manner under a defined action of moisture.Here, the humidity can be set in a targeted manner by means of thetemperature or saturation of the gases, for example, coming into contactwith the membrane, e.g. air, nitrogen, carbon dioxide or other suitablegases, or water vapour. The treatment time is dependent on theparameters selected above.

Furthermore, the treatment time is dependent on the thickness of themembrane.

In general, the treatment time is from a few seconds to minutes, forexample under the action of superheated steam, or up to a number of fulldays, for example in air at room temperature and low relativeatmospheric humidity. The treatment time is preferably in the range from10 seconds to 300 hours, in particular from 1 minute to 200 hours.

If the partial hydrolysis is carried out at room temperature (20° C.)using ambient air having a relative atmospheric humidity of 40-80%, thetreatment time is from 1 to 200 hours.

The membrane obtained in step iv) can be made self-supporting, i.e. itcan be detached from the support without damage and subsequentlyprocessed further directly, if appropriate.

The treatment in step iv) leads to hardening of the coating. If themembrane is formed directly on the electrode, the treatment in step D)is continued until the coating has a hardness sufficient to be able tobe pressed to form a membrane-electrode unit. A sufficient hardness isensured when a membrane treated in this way is self-supporting. However,a lower hardness is sufficient in many cases. The hardness determined inaccordance with DIN 50539 (microhardness measurement) is generally atleast 1 mN/mm², preferably at least 5 mN/mm² and very particularlypreferably at least 50 mN/mm², without this implying a restriction.

The concentration and amount of phosphoric acid and thus theconductivity of the polymer membrane of the invention can be adjustedvia the degree of hydrolysis, i.e. the time, temperature and ambienthumidity. According to the invention, the concentration of phosphoricacid is reported as mole of acid per mole of repeating unit of polymer.For the purposes of the present invention, a concentration (mole ofphosphoric acid per mole of repeating units of the formula (III), i.e.polybenzimidazole) of from 10 to 80, in particular from 12 to 60, ispreferred. Such high degrees of doping (concentrations) can be obtainedonly with great difficulty or not at all by doping of polyazoles withcommercially available orthophosphoric acid.

The thickness of the barrier layer of a multilayer polymer electrolytemembrane according to the invention is generally not critical as long asthis layer has a sufficient barrier action against mineral acids. Thebarrier action can be determined via the amount of mineral acid whichcan be leached out by means of water. According to a particular aspectof the present invention, not more than 10%, preferably not more than5%, of the mineral acid goes over into the aqueous phase during a periodof one hour. These values are based on the weight of mineral acid or theweight of the sheet-like material doped with the mineral acid, with thearea which is in contact with water being in each case employed forcalculating the value.

In a particular embodiment of the present invention, the thickness ofthe barrier layer is less than 10 μm, preferably from 1 to 8 μm andparticularly preferably from 2 to 6 μm. Such barrier layers have theadvantage of a relatively low resistance.

In a further embodiment of the present invention, the thickness of thebarrier layer is at least 10 μm and is preferably in the range from 10μm to 30 μm. Such barrier layers advantageously have a particularly highbarrier action and also a high stability.

The thickness of the barrier layer can be measured by means of scanningelectron microscopy (SEM). Here, the thickness of the barrier layer isthe mean of the thickness obtained via the ratio of area to length ofthe barrier layer.

The barrier layer according to the invention is preferably acation-exchange material. This cation-exchange material allows protonsbut not anions such as phosphate anions to be transported. To improveadhesion, block copolymers comprising components of thepolymerelectrolyte membrane and the cation-exchange membrane can also beused at the interface between polymer electrolyte membrane andcation-exchange material.

This barrier layer can be joined (laminated) in the form of a separatefilm, preferably self-supporting, to the doped polymer membrane or thedoped polymer blend membrane.

Furthermore, the barrier layer can be formed by applying a layer to thedoped membrane and/or the electrode. For this purpose, it is possible,for example, to apply a mixture comprising cation-exchange material or aprecursor material to the membrane and/or the electrode. Suitableprocesses include, inter alia, casting, spraying, doctor blade coatingand/or extrusion.

The barrier layer can also have a gradient. Thus, for example, theconcentration of acid groups can be varied. Such gradients can bemeasured, for example, by means of energy-dispersive X-ray scattering(EDX), location-resolved Raman spectroscopy and location-resolvedinfrared spectroscopy.

In a variant of the present invention, if the cation-exchange materialis present in the form of a self-supporting film, this can also beincorporated as a separate film in an MEU between the doped polymerelectrolyte membrane and the catalyst layer or the electrode (also onboth sides).

It has been found that it is advantageous for the barrier layer to belocated on the cathode side of the polymer electrolyte membrane, sincethe overvoltage is significantly reduced. However, apart from thisembodiment, the barrier layer can also be applied on both sides.

As indicated above, the cation-exchange material is not subject to anysignificant restriction. Preference is given to materials whosecation-exchange capacity is less than 0.9 meq/g, in particular less than0.8 meq/g. The cation-exchange capacity is, according to a particularaspect of the present invention, at least 0.1 meq/g, in particular 0.2meq/g, without this implying a restriction. Preference is given tomaterials whose area swelling in water at 80° C. is less than 20%, inparticular less than 10%. Preference is given to materials whoseconductivity at 80° C. in the moistened state is less than 0.06 S/cm, inparticular less than 0.05 S/cm.

To measure the IEC, the sulphonic acid groups are converted into thefree acid. For this purpose, the polymer is treated in a known mannerwith acid, with excess acid being removed by washing. The sulphonatedpolymer is firstly treated for 2 hours in boiling water. Excess water issubsequently dabbed off and the sample is dried at 160° C. and p<1 mbarin a vacuum drying oven for 15 hours. The dry weight of the membrane isthen determined. The polymer which has been dried in this way is thendissolved in DMSO at 80° C. for 1 hour. The solution is subsequentlytitrated with 0.1 M NaOH. The ion-exchange capacity (IEC) is thencalculated from the consumption of acid to the equivalence point and thedry weight.

At a high current density and temperatures above 100° C., moistening ofthis thin layer is effected by the product water produced at thecathode. When hydrogen-rich reformer gas is used, the moisture presentin the reformer gas is sufficient to moisten the barrier layer. Thus,the system requires no additional moistening at temperatures above 100°C. and high electric power. However, it may sometimes be necessary tomoisten the fuels additionally on start-up or at low temperatures or atlow current densities. The barrier layer applied on the cathode side ispreferably thicker than the barrier layer located on the anode side.

The barrier layer preferably comprises a cation-exchange material. Here,it is in principle possible to use all cation-exchange materials whichcan be processed to form membranes. These are preferably organicpolymers having covalently bound acid groups. Particularly suitable acidgroups include, inter alia, carboxylic acid, sulphonic acid andphosphonic acid groups, with polymers containing sulphonic acid groupsbeing particularly preferred. Methods of sulphonating polymers aredescribed in F. Kucera et. al. Polymer Engineering and Science 1988,Vol. 38, No 5, 783-792.

The cation-exchange materials preferably used as barrier layers cangenerally not be used alone as cation-exchange membranes in fuel cells,since their proton conductivity and swelling is too low and mechanicalstability cannot be ensured because of the low thickness. However, thecation-exchange membranes described in the prior art have been developedwith high ion-exchange capacity, high swelling, high proton conductivityand sufficient thickness to achieve sole use as polymer electrolytemembranes in MEUs.

The most important types of cation-exchange membranes which haveachieved commercial importance for use in fuel cells are describedbelow.

The most important representative is the perfluorosulphonic acid polymerNafion® (U.S. Pat. No. 3,692,569). This polymer can be brought intosolution as described in U.S. Pat. No. 4,453,991 and then used asionomer. Cation-exchange membranes are also obtained by filling a poroussupport material with such an ionomer. As support material, preferenceis given to expanded Teflon (U.S. Pat. No. 5,635,041).

A further perfluorinated cation-exchange membrane can be produced asdescribed in U.S. Pat. No. 5,422,411 by copolymerization oftrifluorostyrene and sulphonyl-modified trifluorostyrene. Compositemembranes comprising a porous support material, in particular expandedTeflon, filled with ionomers consisting of such sulphonyl-modifiedtrifluorostyrene copolymers are described in U.S. Pat. No. 5,834,523.

U.S. Pat. No. 6,110,616 describes copolymers of butadiene and styreneand their subsequent sulphonation to produce cation-exchange membranesfor fuel cells.

A further class of partially fluorinated cation-exchange membranes canbe produced by radiation grafting and subsequent sulphonation. Here, agrafting reaction, preferably using styrene, is carried out on apreviously irradiated polymer film, as described in EP-A-667983 orDE-A-19844645. In a subsequent sulphonation reaction, the side chainsare then sulphonated. Crosslinking can also be carried outsimultaneously with grafting and the mechanical properties can bealtered in this way.

Apart from the above membranes, a further class of nonflourinatedmembranes obtained by sulphonation of high-temperature-stablethermoplastics has been developed. Thus, membranes composed ofsulphonated polyether ketones (DE-A-4219077, WO 96/01177), sulphonatedpolysulphone (J. Membr. Sci. 83 (1993) p. 211) or sulphonatedpolyphenylene sulphide (DE-A-19527435) are known. Ionomers prepared fromsulphonated polyether ketones are described in WO 00/15691.

Furthermore, acid-base blend membranes produced by mixing sulphonatedpolymers and basic polymers as described in DE-A-19817374 or WO 01/18894are known.

To set the ion-exchange capacity for optimal acid retention, acation-exchange membrane known from the prior art can be mixed with apolymer bearing no acid groups or only a small amount of acid groups.Suitable polymers have been described above as blend components, withhigh-temperature-stable polymers being particularly preferred. Thepreparation and properties of cation-exchange membranes comprisingsulphonated PEK and a) polysulphones (DE-A4422158), b) aromaticpolyamides (DE-A-42445264) or c) polybenzimidazole (DE-A-19851498) havebeen described. As an alternative, the sulphonation conditions can bechosen so that a low degree of sulphonation results (DE-A-19959289).

Apart from the cation-exchange membranes mentioned in the prior artwhich are based on organic polymers, the cation-exchange material canalso be made of organic-inorganic composite materials. Such compositematerials are preferably prepared by means of the sol-gel process. Asstarting compounds, use is made of mixtures of metal alkoxides, inparticular siloxanes. These mixtures have a high purity of the startingmaterials and a low viscosity. These liquid precursor mixtures can beapplied to a substrate by means of known technologies, for examplespraying or spin coating, to give very thin and uniformly coveringlayers. Hydrolysis and condensation of the precursor mixtures thenenables solid films to be produced on the surface. To obtain protonconductivity, the organic radicals of the alkoxides containacid-containing groups, in particuar sulphonic acid groups.

The precursor mixtures can likewise contain functional organic groupswhich effect crosslinking of the layer formed and thus a furtherreduction in the permeability to the mineral acid and the fuels.Crosslinking can be carried out after layer formation either thermallyor by irradiation (electron beam, UV, IR, NIR) or by means of aninitiator.

The production of such a composite material is described, for example inElectrochimica Acta volume 37, year 1992, pages 1615-1618. Furthermore,such composite materials are known from G. W. Scherer, C. J. Brinker,Sol-Gel-Science, Academic Press, Boston, 1990.

One group of preferred compounds can be represented by the formula (A)(RO)_(y)(R¹)_(z)M-X_(a)  (A)where

-   y is 1, 2 or 3, preferably 3,-   z is 0 or 1, preferably 0 and-   a is 1 or 2, preferably 1, and-   R and R¹ are each, independently of one another, hydrogen, a linear    or branched alkyl, alkenyl, cycloalkyl or cycloalkenyl radical    having from 1 to 20, preferably from 1 to 8, carbon atoms, or an    aromatic or heteroaromatic group having from 5 to 20 carbon atoms,-   M is an element selected from among Si, Zr, Ti, preferably Si, and    the radicals X are each, independently of one another, a linear or    branched alkylene or cycloalkylene group having from 1 to 20,    preferably from 1 to 8, carbon atoms or an aromatic or    heteroaromatic group having from 5 to 20 carbon atoms and bearing at    least one sulphonic acid or phosphonic acid.

The radicals R, R¹ and X can have further substituents, in particularhalogens such as fluorine atoms. The group X is preferably a radical ofone of the formulae Ph-SO₃H, C_(n)H_(2n)—SO₃H, C_(n)F_(2n)—SO₃H dar,where Ph is phenyl and n is an integer from 1 to 20. The group R ispreferably a radical of the formula C_(n)H_(2n+1), where n is from 1 to3.

Preferred compounds are, in particular, hydroxysilyl acids, which areknown per se and are described, for example, in DE 100 61 920, EP 0 771589, EP 0 765 897 and EP 0 582 879.

Preferred hydroxysilyl acids can be represented by the formula B or C[(RO)_(y)(R²)_(z)Si—{R¹—SO₃ ⁻}_(a)]_(x)M^(x+)  (B)[(RO)_(y)(R²)_(z)Si—{R¹—O_(b)—P(O_(c)R₃O₂ ⁻}_(a)]_(x)M^(x+)  (C)where M is H⁺, NH₄ ⁺ or a metal cation having a valence x of from 1 to4, and y=1 to 3, z=0 to 2 and a=1 to 3, with the proviso that y+z=4−a,

-   b and c are 0 or 1, R and R² are identical or different and are each    methyl, ethyl, propyl, butyl radicals or H and-   R³ is M or a methyl, ethyl, propyl, butyl radical, and-   R¹ is a linear or branched alkyl or alkylene group having from 1 to    12 carbon atoms,-   a cycloalkyl group having from 5 to 8 carbon atoms or a unit of one    of the general formulae

where n and m are each a number from 0 to 6.

Preferred hydroxysilyl acids or precursors (derivatives) thereof aretrihydroxysilylethylsulphonic acid, trihydroxysilylphenylsulphonic acid,trihydroxysilylpropylsulphonic acid,trihydroxysilylpropylmethylphosphonic acid anddihydroxysilylpropylsulphonic diacid or salts thereof.

The structure of cation-exchange material can be set precisely byappropriate choice of trihydroxysilyl acid (network former),dihydroxysilyl acid (chain former) and monohydroxysilyl acid (chain end)and by addition of further sol formers. Suitable sol formers are, forexample, the hydrolyzed precursors of SiO₂, Al₂O₃, P₂O₅, TiO₂ or ZrO₂.Preferred compounds include, inter alia, tetramethoxysilane,tetraethoxysilane, triethoxyvinylsilane, trimethoxyvinylsilane,triethoxypropenylsilane and trimethoxypropenylsilane.

As substrates for the deposition of the barrier layer, it is possible touse either a film of the basic polymer, a polymer electrolyte membranedoped with mineral acid or an electrode coated with noble metalcatalyst.

In one variant of the invention, the barrier layer is deposited on anelectrode.

According to a particular aspect of the present invention, the materialfrom which the barrier layer is produced is chemically compatible withthe sheet-like material doped with at least one mineral acid, so thatgood adhesion of the barrier layer to the sheet-like material isachieved. Accordingly, when a polyazole film is used, particularpreference is given to using organic cation-exchange polymers to whichthe abovementioned polyazoles have good adhesion. Such polymers include,in particular, sulphonated polysulphones, polyether ketones and otherpolymers which have aromatic groups in the main chain. When inorganicmaterials are used, good adhesion to the organic or inorganic supportscan be achieved by choice of appropriate functional groups.

When inorganic sheet-like materials are used, preference is accordinglygiven to using the abovementioned inorganic layers which can beobtained, for example, by hydrolysis of hydroxysilyl acids.

The multilayer electrolyte membranes of the invention display, takinginto account the barrier layer, excellent conductivity and performance.

The proton conductivity of preferred multilayer electrolyte membranes attemperatures of 120° C. is preferably at least 0.1 S/cm, in particularat least 0.11 S/cm, particularly preferably at least 0.12 S/cm. Thisconductivity is also achieved at temperatures of 80° C.

A membrane according to the invention can be moistened at lowtemperatures. For this purpose, for example, the compound used as energysource, for example hydrogen, can be provided with a proportion ofwater. However, the water formed by the reaction is in many casessufficient to achieve moistening.

The specific conductivity is measured by means of impedance spectroscopyin a 4-pole arrangement in the potentiostatic mode using platinumelectrodes (wire, 0.25 mm diameter). The distance between thecurrent-collecting electrodes is 2 cm. The spectrum obtained isevaluated using a simple model consisting of a parallel arrangement ofan ohmic resistance and a capacitor. The specimen cross section of themembrane doped with phosphoric acid is measured immediately beforemounting of the specimen. To measure the temperature dependence, themeasurement cell is brought to the desired temperature in an oven andthe temperature is regulated by means of a Pt-100 resistance thermometerpositioned in the immediate vicinity of the specimen. After thetemperature has been reached, the specimen is maintained at thistemperature for 10 minutes before the start of the measurement.

The polymer membrane of the invention displays improved materialsproperties compared to the previously known doped polymer membranes.Owing to the low methanol permeability, the multilayer membranes can beused, in particular, in direct methanol fuel cells.

The crossover current density in a liquid direct methanol fuel celloperated at 90° C. using 0.5 M methanol solution is preferably less than100 mA/cm², in particular less than 70 mA/cm², particularly preferablyless than 50 mA/cm² and very particularly preferably less than 10mA/cm². The crossover current density in a gaseous direct methanol fuelcell operated at 160° C. using 2 M methanol solution is preferably lessthan 100 mA/cm², in particular less than 50 mA/cm², very particularlypreferably less than 10 mA/cm².

To determine the crossover current density, the amount of carbon dioxideliberated at the cathode is measured by means of a CO₂ sensor. Thecrossover current density is calculated from the measured value of theamount of CO₂, as described by P. Zelenay, S. C. Thomas, S. Gottesfeldin S. Gottesfeld, T. F. Fuller “Proton Conducting Membrane Fuel CellsII” ECS Proc. Vol. 98-27 pp. 300-308.

The invention further provides for the preferred use of the multilayerelectrolyte membrane of the invention or the coated electrode in amembrane-electrode unit (MEU) for a fuel cell.

The MEU comprises at least one multilayer electrolyte membrane accordingto the invention and two electrodes between which the multilayerelectrolyte membrane is located in a sandwich-like arrangement.

The electrodes each have a catalytically active layer and a gasdiffusion layer for bringing a reaction gas to the catalytically activelayer. The gas diffusion layer is porous so that reactive gas can passthrough it.

The multilayer electrolyte membrane of the invention can be used aselectrolyte membrane in electrochemical processes. In addition, it ispossible to produce the electrolyte membrane or an intermediatestructure for an MEU with one or both catalytically active layers.Furthermore, the MEU can also be produced by fixing the gas diffusionlayer to the intermediate structure.

The present invention further provides a fuel cell system comprising aplurality of different MEUs of which at least one contains a multilayermembrane according to the invention.

A membrane-electrode unit according to the invention displays asurprisingly high power density. In a particular embodiment, preferredmembrane-electrode units produce a current density of at least 0.1A/cm², preferably 0.2 A/cm², particularly preferably 0.3 A/cm². Thiscurrent density is measured in operation using pure hydrogen at theanode and air (about 20% by volume of oxygen, about 80% by volume ofnitrogen) at the cathode at atmospheric pressure (1013 mbar absolute,with open cell outlet) and a cell voltage of 0.6 V. Here, particularlyhigh temperatures in the range 150-200° C., preferably 160-180° C., inparticular 170° C., can be used.

The power densities mentioned above can also be achieved at a lowstoichiometry of the fuel gases on both sides. According to a particularaspect of the present invention, the stoichiometry is less than or equalto 2, preferably less than or equal to 1.5, very particularly preferablyless than or equal to 1.2.

EXAMPLES 1 to 6

Production of Cation-Exchange Membranes:

To produce cation-exchange membranes, the following stock solutions wereprepared.

-   a) 10 wt % of PES (Ultrason E 7020 P) in NMP-   b) 17 wt % of sPEK (degree of sulphonation: 50.3%) in NMP

The solutions were mixed in the ratios indicated in Table 1 and appliedby means of a doctor blade coater (50 μm). The films were subsequentlydried in an oven at 120° C. for 11 hours. The thickness of the filmsproduced is 20-25 μm. The polymers used for producing the membrane areshown in Table 1.

TABLE 1 PES sPEK [% by weight] [% by weight] Example 1 0 100 Example 220 80 Example 3 30 70 Example 4 40 60 Example 5 50 50 Example 6 60 40

The specific conductivity is measured by means of impedance spectroscopyin a 4-pole arrangement in the potentiostatic mode using platinumelectrodes (wire, 0.25 mm diameter). The distance between thecurrent-collecting electrodes is 2 cm. The spectrum obtained isevaluated using a simple model consisting of a parallel arrangement ofan ohmic resistance and a capacitor. The specimen cross section of thesulphonated PEK membranes and sulphonated PEK blend membranes ismeasured after swelling in water at 80° C. for 1 hour prior to mountingof the specimen. To measure the temperature dependence and formoistening, the measurement cell is rinsed with heated water. Beforecommencement of the experiment, the cell is maintained at 80° C. for 30minutes and the conductivity measurement is then commenced. Cooling iscarried out at 1 K/min. Before the start of each new measurement, thedesired temperature is then maintained for 10 minutes.

Table 2 shows the results of the conductivity measurements onsulphonated PEK membranes and sulphonated PEK blend membranes.

TABLE 2 Conductivity values of sulphonated PEK membranes and sulphonatedPEK blend membranes (proportion of PES blend component in per cent byweight) for use as barrier layer for phosphoric acid T (° C.) Ex. 1 Ex.2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 80 0.196 0.160 0.150 0.149 0.046 0.035 700.181 0.148 0.139 0.137 0.042 0.031 60 0.164 0.136 0.125 0.125 0.0370.028 50 0.150 0.124 0.113 0.112 0.032 0.025 40 0.133 0.110 0.099 0.0980.027 0.022 30 0.116 0.096 0.086 0.085 0.023 0.018 22 0.105 0.086 0.0770.074 0.020 0.016

The conductivity and barrier action of the cation-exchange membrane forphosphoric acid depend strongly on the content of acid groups expressedby the ion-exchange capacity (IEC).

To measure the IEC, the sulphonated polymer or the sulphonated blendmembrane is firstly treated in boiling water for 2 hours. Excess wateris subsequently dabbed off and the specimen is dried at 160° C. in avacuum drying oven at p<1 mbar for 15 hours. The dry weight of themembrane is then determined. The polymer which has been dried in thisway is then dissolved in DMSO at 80° C. for 1 hour. The solution issubsequently titrated with 0.1 M NaOH. The ion-exchange capacity (IEC)is then calculated from the consumption of the acid to the equivalencepoint and the dry weight.

To determine the swelling behaviour, the sulphonated membranes or blendmembranes are swollen at 80° C. for 2 hours and the increase in area isdetermined.

Table 3 shows the ion-exchange capacity of a sulphonated PEK membrane(0% of PES) and blend membranes of sulphonated PEK and various contentsof PES.

TABLE 3 Ion-exchange capacity and swelling at 80° C. of a sulphonatedPEK membrane (0% of PES) and blend membranes of sulphonated PEK andvarious contents of PES T = 80° C. IEC (meq/g) Swelling (%) Example 12.06 156 Example 2 1.71 124.6 Example 3 1.34 61.6 Example 4 1.03 41.7Example 5 0.8 8.6 Example 6 0.59 2

To measure the barrier action of the cation-exchange membranes for theexample of membranes doped with phosphoric acid, the following procedureis employed:

A cation-exchange membrane having a diameter of 7 cm in the dry state isfirstly stamped out. This membrane is subsequently dipped into 300 ml ofwater and the pH change is measured as a function of time. In the caseof these membranes, the pH can, owing to the material selected, decreasebecause of residues of free acid from the sulphonation reaction. Sinceeach membrane has a different content of acid groups, this blank has tobe measured for each individual membrane.

Such a membrane is subsequently clamped into the measurement apparatusagain and an acid-doped membrane is placed on top. To carry out doping,a PBI film having an initial thickness of 50 μm is placed in 85%phosphoric acid for at least 72 hours at room temperature. A piece ofthis acid-doped membrane having a diameter of 3 cm is stamped out andimmediately laid on the cation-exchange membrane. The sandwich producedin this way is then placed in a glass beaker filled with 300 ml of waterand the pH change is measured over 15 hours at room temperature (20°C.). A schematic structure of the measurement apparatus is shown inFIG. 1. The result obtained in this way is shown graphically in FIG. 2.

The negative values in FIG. 3 after correction of the blank can beexplained by the loss of acid from the cation-exchange membrane (blank)itself being greater than the passage of phosphoric acid through thecation-exchange membrane.

In FIG. 4, the measurement of the amount of acid which has passedthrough the barrier layer and that which has been retained by thebarrier layer is demonstrated beyond doubt.

The results demonstrate that the use of cation-exchange materials asbarrier layer leads to a surprisingly clear reduction in the liberationof mineral acid.

It can, surprisingly, be seen from the results obtained that preferredcation-exchange membranes according to the invention in the moistenedstate at 80° C. display a conductivity of <0.06 S/cm, in particular<0.05 S/cm.

Preferred cation-exchange membranes according to the invention have anIEC value of less than 0.9 meq/g. The swelling of preferredcation-exchange membranes is less than 20% at 80° C.

It has surprisingly been found that the use of the membrane of theinvention, i.e. the membrane provided with a barrier layer, having anion-exchange capacity of less than 0.9 meq/g and a swelling in water ofless than 10% at 80° C. leads to a particularly significant reduction inthe passage of phosphoric acid and the acid concentration does not goabove 0.0005 mol/l over a period of 15 hours.

EXAMPLE 7

Production of an Ultrathin Cation-Exchange Membrane as Barrier Layer onthe Membrane Surface:

Production of PBI Film:

A 50 μm thick film of a 15% strength by weight polybenzimidazole (PBI)solution in DMAc was spread by means of a doctor blade and dried at 120°C. in an oven for 12 hours.

Preparation of the Spray Solution:

A 10% strength by weight solution of PES (Ultrason E 7020) and sPEK(degree of sulphonation: 50.3%) in DMAc was prepared, with the weightratio of PES to sPEK being 60:40.

Coating:

To apply the coating, a glass plate was placed on a hotplate and heatedto 150° C. After this temperature had been reached, the PBI film waslaid on the glass plate. As soon as the film had drawn flat onto theglass plate, a metal template was placed on top. The spray solution wassprayed onto the film surface a number of times by means of an airbrush.The solvent was evaporated after each spraying step. The metal templatewas then taken off and the sprayed region was cut out. The thickness ofthe coating was 4-5 μm.

The coated polyazole film is clamped in place with the coated sideuppermost as shown in FIG. 1 and then dipped into a glass beaker filledwith 100 ml of water. In this configuration, the underside is in contactwith water while 0.5 ml of phosphoric acid is applied to the oppositeside.

The change in the pH was observed over a period of 50 hours. Forcomparison, a polyazole film without a barrier layer was subjected tothe same test.

The results obtained are shown in FIG. 5, and the effectiveness of thethin barrier layer can clearly be seen.

1. A multilayer proton-conducting electrolyte membrane comprising: a) asheet-like material doped with one or more mineral acids, and b) atleast one barrier layer which covers at least one of the two surfaces ofthe sheet-like material, wherein the multilayer electrolyte membrane hasproton conductivity of at least 0.1 S/cm at 120° C.
 2. The electrolytemembrane of claim 1, wherein the sheet-like material comprises amaterial selected from the group consisting of a basic polymer, amixture of one or more basic polymers with other polymers, and achemically inert support.
 3. The electrolyte membrane of claim 2,characterized in that the basic polymer has at least one nitrogen atomin a repeating unit.
 4. The electrolyte membrane of claim 2,characterized in that the basic polymer contains at least one aromaticring having at least one nitrogen atom.
 5. The electrolyte membrane ofclaim 4, characterized in that the basic polymer is a polyimidazole, apolybenzimidazole, a polybenzothiazole, a polybenzoxazole, apolytriazole, a polyoxadiazole, a polythiadiazole, a polypyrazole, apolyquinoxaline, a poly(pyridine), a poly(pyrimidine), or apoly(tetrazapyrene).
 6. The electrolyte membrane of claim 2,characterized in that a mixture of one or more basic polymers with afurther polymer is used.
 7. The electrolyte membrane of claim 1,characterized in that the mineral acid is phosphoric acid or sulphuricacid.
 8. The electrolyte membrane of claim 1, characterized in that thebarrier layer is a cation-exchange material.
 9. The electrolyte membraneof claim 8, characterized in that the cation-exchange material has anion-exchange capacity (IEC) of less than 0.9 meq/g.
 10. The electrolytemembrane of claim 8, characterized in that the cation-exchange materialhas an area swelling in water at 80° C. of less than 20%.
 11. Theelectrolyte membrane of claim 8, characterized in that thecation-exchange material has a conductivity of less than 0.06 S/cm at80° C. in a moistened state.
 12. The electrolyte membrane of claim 1,characterized in that the barrier layer has a thickness of from 10 to 30μm.
 13. The electrolyte membrane of claim 1, characterized in that thebarrier layer has a thickness of less than 10 μm.
 14. The electrolytemembrane of claim, characterized in that the barrier layer applied on acathode side is thicker than the barrier layer located on an anode side.15. The electrolyte membrane of claim 1, characterized in that thebarrier layer is a cation-exchange material based on an organic polymeror an organic-inorganic composite material having covalently bound acidgroups selected from the group consisting of carboxylic acids, sulphonicacids, and phosphonic acids.
 16. A multilayer proton-conductingelectrolyte membrane comprising: a) a sheet-like material doped with oneor more mineral acids, and b) at least one barrier layer which covers atleast one of the two surfaces of the sheet-like material, said barrierlayer comprising a cation-exchange material, wherein the multilayerelectrolyte membrane has proton conductivity of at least 0.1 S/cm at120° C.